Instructables
Many of you are probably aware of the "joule thief" circuit. If you aren't, it's a very simple voltage booster that is normally used to power an LED off of a mostly dead AA battery, but it can be used for other applications where a voltage boost is needed. One such application is charging a battery off of a battery with a smaller volatge rating. There are designs out there for battery chargers using a 9 volt battery and then step down the voltage to the level of the battery to be charged. But this circuit uses any single battery with a rating of 1.5 volts, such as a AA, C, or D battery, and bumps the voltage up to a level that can charge a battery with a voltage rating of 5 volts or lower. This curcuit is extremely simple, dirt cheap, and can be built in an afternoon using parts taken from other old devices. 
 
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Step 1: Parts and Pieces

Picture of Parts and Pieces
Parts for the charger include:
2N3904 NPN transistor
1K ohm resistor
Toroid transformer core
Magnet wire
Diode
SPST Switch (not pictured)

A word about some of the parts:
The toroid core can be any size, but nothing fancy or big is needed. A small toroid is probably better just because it is more compact. Only one toroid is used in the circuit, but two are pictured for size reference. 
The diode can be just about any diode, but the lower the forward voltage drop, the better. Germanium diodes work the best. To find the forward voltage drop, simply hook a diode up to a battery, and measure the difference in voltage with and without the diode in the circuit. If the voltage drop is about equal to the voltage being supplied, make sure the diode isn't hooked up backwards.
GenesisMaster6 months ago

Is it possible to add a charge indicator circuit like this one on the charger?

http://www.instructables.com/id/9v-battery-status-...

Wait.. what exact voltage that can out from that device?
I'm sure you've found an answer by now, but if not/anyone else reading this gets curious:

The output voltage is dependent on the voltage needed to make the magnetic field dissipate. This is a "self regulating" boost converter, which essentially makes a magnetic field then turns itself off, leaving nowhere for that field to go. As it tries to equalize, it will force itself through the breakdown voltage of the transistor (typically 30-50v). However, if you have a few LEDs that only need 10v to run, it will take that magnetic field and turn it into 10V (with the equivalent lesser current running through the LEDs). This will happen ~50k times a second.

So effectively, this takes .7+ volts (Vi) at Ii amps to be <(reverse breakdown voltage) (Vo) at (Vi*Ii)/(Vo)*efficiency (40-90%).
humm.. Thanks to your answer.. so now I know.
Now I will make my own design for portable charger..
so coming soon from now.... hehehe :)
waterlubber2 years ago
Hmm...
How about a joule thief that cranks up a hydroelectric fuel cell? Than, we could convert water into power! LED power! Mwa hahahaha!
error323 years ago
I would be interested to see you measure the charging current here
JoeBeau (author)  error323 years ago
If i am reading my meter right, i am getting 13ma for a AA, 15ma for a C, and 18ma for a D battery. This is with the meter hooked directly into the voltage out terminals.
cool! could you post a video on youtube? and whats ther current draw fro the battery? i was thinking about using a tree battery and this circuit to make tree lights :D
JoeBeau (author)  iamdarkyoshi2 years ago
Im sorry, but whats a tree battery? and when you refer to the current draw, are you talking about the charger drawing out of the battery? Cause the output current without a load is actually listed in the comment above yours.
a tree battery was something i found on the internet. pound a few pennies into a tree and parallel them, and parallel galvanized nails a foot lower on the tree. i get about .3 volts off of it, and i thought if i put some in series, i could make tree powered walkway lights! lol that would be just hilarious!
JoeBeau (author)  iamdarkyoshi2 years ago
That sounds pretty cool. It would be interesting to see what kind of voltages can be acheived using a joule thief running off a tree battery. I dont know that it would be enough though to power a whole walkway though. Maybe use the tree batteery power to charge some batteries that then power the walkway lights
thts what i was gonna do. but... maybe one joule theif to get tree up to 1.5 volt for battery, and another to get it to 4.5. my walkways are really weird anyway, and ther not very long O.o
JoeBeau (author)  iamdarkyoshi2 years ago
I've tried running one joule thief of another, but t doesnt work directly. What you need to do is have the first one charge a battery that the second one runs off of.
Or a BIG capacitor.
ionic JoeBeau2 years ago
Provided you keep the voltage above the charging batteries nominal voltage and 13mA is constant, how many dead AA's would it take to charge another AA battery? For a 2000mAH AA battery with 200mA charge current it will take 12hrs - 14hrs to fully charge it. At 13mA charge current it will take you roughly 15X as long
and you will need a hand full of batteries to do it as well.

The circuit works nice for a White LED power source, but I would rather call the device in this configuration more of a Trickle Charger, if that. Do you have any data on how long it takes to charge a particular battery and how many dead AA's it takes to do the job?
what diode you are using ?
JoeBeau (author)  echuzzzzcute2 years ago
A cat-whisker diode. The reason it is being used is that it has a very low voltage drop. Many of these diodes dont have numbers printed on them unfortunately. Old audio, radio, or television equipment is a good source of these.
francisroan2 years ago
is it ok if i use the big toroid?
JoeBeau (author)  francisroan2 years ago
Any size toroid can be used. Longer cylindrical ones work the best. Look for them in the RF chokes used on power cables, or in CRT screens.
how many windings? and can i use normal wire like abt 3mm thick its thicker than urs and doesnt look pretty after winding and tried the whole thing with the ciruit and it doesnt workhelp!!!!
JoeBeau (author)  francisroan2 years ago
The number of windings isnt too critical- somewhere between 8 and 15 is good. It is important to make sure the windings are neat however, because if the wires cross eachother, he circuit's efficency is greatly reduced. One of the biggest reasons for the circuit not to work is that the toroid is hooked up out of phase. Step 4 details how the transformer should be hooked up.
Munchys3 years ago
I wanted to put a solar panel to charge batteries and when it becomes night the batteries light up the leds :D For those little things outside around walkways
JoeBeau (author)  Munchys3 years ago
I've seen it done with the joule theif used to charge the battery, or the joule theif used to power the LEDs